Nuclear magnetic resonance (NMR) imaging has been known and used as a noninvasive diagnostic tool by physicians for some time. One technique for obtaining diagnostic image includes building an image out of a series of planar images to obtain the entire image and then reconstructing the desired slice for visualization purposes.
Conventional multislice imaging methods involves applying an initiation sequence, followed by an acquisition sequence. The initiation sequence may include one or more 1.degree. to 180.degree. saturation pulses, 180.degree. inversion pulses, 90.degree.-180.degree. partial inversion pulses, or combinations thereof. Typically, 90.degree. saturation pulses are applied in the initiation sequence in time of flight flow imaging or for flow artifact suppression, while 180.degree. inversion pulses are applied for more routine image scanning.
After application of an inversion pulse the magnetization of the sample is reversed but recovers exponentially with a time constant T.sub.1, the longitudinal relaxation time. During the acquisition sequence, at least one spin echo sequence or gradient echo sequence is applied, and the "echo" emitted from the sample is detected and analyzed. The spin echo or gradient echo may include a 1.degree. to 180.degree. saturation pulse to reduce artifacts arising from blood flow motion. The amplitude of the induced echo depends on T.sub.2, the spin-spin relaxation time. In some cases to obtain better tissue contrast, multiple echo sequences may be applied. The signal produced and measured with either a spin echo or gradient echo is spatially encoded to produce a sample image. Typically, spatial encoding comprises application of three magnetic field gradients, a slice selection gradient, phase encoding gradient, and a readout gradient, and Fourier analysis of the accumulated data. Whole-body NMR imagers typically require repeated RF irradiations to form an image.
Due to its ability to give good T.sub.1 contrast differentiation and selective suppression capability, for example fat suppression, inversion recovery imaging is useful in many clinical applications. However, because of the long recovery times required upon application of an inversion pulse, its use generally results in obtaining inversion recovery images for only a limited number of sample slices over a given time period.
During the time a patient undergoes NMR imaging, they are inaccessible to the physician. A minimum residence time in the NMR image scanner is desirable for critically ill or claustrophobic patients where immediate intervention may become necessary. Moreover, a minimum patient residence time in the NMR image scanner is desirable as it allows for increased throughput in an expensive piece of equipment. By decreasing patient residence time, more patients can undergo NMR imaging in a given time period enabling a medical institution or medical office to more quickly recoup the high capital cost of purchasing NMR image scanning equipment.
In this regard, two multislice acquisition schemes have been proposed for more efficient NMR imaging.
The first method is similar to the conventional multislice acquisition scheme for saturation recovery imaging described in Crooks, L. E.; Ortendahl, D. A.; Kaufman, L., et al.; "Clinical efficiency of nuclear magnetic resonance imaging" Radiology, 146:123-128; 1983. In this method, illustrated in FIG. 1, the combination of an inversion RF pulse initiation sequence and acquisition sequence for each slice is repeated for multislice acquisition during each repetition series. An acquisition sequence composed of selective 90.degree. and 180.degree. RF pulses is selected as an example. FIG. 1 shows the pulse sequence, where selective RF pulses and echo signals are shown as white Gaussian-function-shaped pulses and black triangles, respectively. Small and large white pulses represent respectively 90.degree. and 180.degree. RF pulses. T.sub.r is the repetition time between encodings of Fourier imaging sequence (between views in case of projection-reconstruction-type imaging, or between measurements for multiple measurement case), T.sub.s is the interval between the acquisitions of different slices. T.sub.a is the minimum length of the acquisition pulse sequence (possibly multiecho sequence) for one slice and one encoding (including some overhead for data transfer and system control and not including the inversion RF pulse and recovery time), T.sub.p is the width of the slice-selective inversion RF pulse (including the time for any artifact suppression spoiling gradient if necessary), and T.sub.i is the recovery time from spin inversion which is defined as the time interval between the inversion pulse and the acquisition sequence for a given slice. As can be seen from FIG. 1, the minimum T.sub.s for each slice is T.sub.i +T.sub.a. The maximum number of slices that can be acquired using this sequence, N.sub.1, can be expressed as EQU N.sub.1 =Int [T.sub.r /(lT.sub.i +T.sub.1)], (1)
where Int(x) is defined as the maximum integer not greater than x. Although this method is efficient when T.sub.i is short, it is not useful otherwise.
A second conventionally used method is a pulse sequence where the inversion-recovery time is used for the inversion pulses for other slices and is illustrated in FIG. 2. The maximum number of slices which can be acquired by using this method can be calculated as follows. Since the acquisition sequence corresponding to each inversion RF pulse takes at least T.sub.a, the maximum number of inversion pulses that can be placed during one inversion-recovery time (T.sub.i) with this interval T.sub.a is (T.sub.i -T.sub.p)/(T.sub.a). The acquisition sequence for these additional slices are applied after the acquisition sequence of the first slice. The total number of slices for one series of RF and acquisition sequences composed as this, N, can be expressed as EQU N=Int [(T.sub.i -T.sub.p)/T.sub.a ]+1. (2)
The acquisition time of N slices are (T.sub.i +N * T.sub.a). Although only one series for N slices is shown in FIG. 2, this can be repeated more than once, i.e. for additional slices, if T.sub.r is long enough. The number of the series in one T.sub.r, M, can be expressed as EQU M=Int [T.sub.r /(T.sub.iI +N*T.sub.a)]. (3)
Thus the total number of slices, N.sub.2, can be expressed as EQU N.sub.2 =N*M. (4)
This method is useful when inversion-recovery time is long such that many inversion pulses can be applied during one recovery time. Although this method utilizes the inversion-recovery time for the acquisition of other slices, there still remains some unused time between RF pulses for each series. Thus, none of the existing multislice data acquisition methods for inversion-recovery magnetic resonance imaging provide an optimized sequence for efficient time utilization.